A residual stress in the tensile direction (tensile residual stress) possibly applies to the inner surface in the vicinity of a welded part of a pipe due to a heat history in welding. The tensile residual stress may cause generation and development of stress corrosion cracking in a high temperature water pipe made of austenitic stainless steel. Therefore, when a tensile residual stress applied to the inner surface in the vicinity of a welded part is improved to the compressive direction, or hopefully changed to a compressive residual stress, damage of a pipe due to stress corrosion cracking can be inhibited.
A method of rapid cooling of the inner surface after heating a pipe is one of the methods improving a tensile residual stress applied to the inner surface in the vicinity of a welded part of the pipe to the compressive direction. According to the method for improving a tensile residual stress to the compressive direction by rapid cooling of the inner surface after heating a pipe, the residual stress can be improved to the compressive direction even in a small diameter pipe which is with thin thickness and is hard to impart a great temperature difference between the inner and outer surfaces of the pipe because a temperature difference between the inner and outer surfaces is adjustable by adjusting the heating temperature.
Representative methods for improving a tensile residual stress applied to the inner surface to the compressive direction by rapid cooling of the inner surface after heating a pipe are disclosed in Japanese Published Unexamined Patent Application No. 54-94415, Japanese Patent No. 4196755, and Japanese Published Unexamined Patent Application No. 2005-320626. These documents describe methods for improving a tensile residual stress in the inner surface of a pipe to the compressive direction by supplying a coolant to the inside of a pipe after heating the pipe from the outer surface to a predetermined temperature and providing the inner surface of the pipe with a tensile yield stress by a thermal stress generated by a temperature difference between the inner and outer surfaces of the pipe.
Japanese Published Unexamined Patent Application No. 54-94415 discloses a method in which a tensile residual stress. applied to the inner surface of a pipe is relaxed or changed to a compressive stress by evenly heating entire group of pipes, thereafter allowing cooling material to flow into the pipe, thereby imparting a temperature difference between the inner and outer surfaces, and providing the inner surface with a tensile yield stress.
Japanese Patent No. 4196755 discloses a variation in a residual stress when a pipe after welding is heated to 200-900° C. (degrees Celsius), soaked for 1 hour, and air-cooled or water-cooled on the inner surface in order to reduce the residual stress. It also discloses that the reducing effect of the residual stress in the inner surface in the axial direction is greater as the heating temperature is higher and that the reducing effect of the residual stress is greater in water cooling of the inner surface than in air cooling. It is after the heating temperature exceeds approximately 600° C. that the residual stress in the inner surface in the axial direction becomes the compressive residual stress under the condition that the cooling method is water cooling of the inner surface.
Japanese Published Unexamined Patent Application No. 2005-320626 discloses a method for improving a tensile residual stress applied to the inner surface of a pipe to the compressive direction by uniformly heating the pipe and thereafter allowing a coolant to flow into the pipe, and thereby imparting a temperature difference between the inner and outer surfaces. It also discloses a method for specifying a minimum value of the cooling water quantity for each inside diameter of pipes as a method for construction management.
As described above, in order to inhibit damage of a pipe made of austenitic stainless steel due to stress corrosion cracking, it is necessary to improve a tensile residual stress generated by a heat history in welding to the compressive direction or hopefully to change the tensile residual stress to a compressive residual stress.
According to the methods for improving a residual stress to the compressive direction by rapid cooling of the inner surface after heating a pipe shown in the above-mentioned three documents, as shown in Japanese Patent No. 4196755, the reducing effect of the residual stress for the inner surface of a pipe improves as the heating temperature of a pipe is higher. This is because, as the heating temperature of the pipe rises, a temperature difference between the inner and outer surfaces of the pipe increases when water-cooling the inner surface. Since a generated thermal stress increases as the temperature difference increases, the amount of plastic deformation in the tensile direction generated in the inner surface of the pipe increases and the reducing effect of the residual stress improves.
However, it takes a long time to heat a pipe to a high temperature at a uniform temperature. Also, when a pipe is held at a high temperature for a long time, material deterioration, such as embrittlement of material and precipitation of carbide, may possibly occur according to the temperature band. For example, it is known that σ embrittlement occurs in the temperature band of 600° C.-900° C. in austenitic stainless steel. Possibly, 475° C. embrittlement may occur at a temperature in the vicinity of 475° C. even in austenitic stainless steel since a ferrite phase is included in a weld metal in the welded part. Accordingly, from the viewpoint of shortening the construction time and reducing material deterioration, it is preferable that the heating temperature of the pipe should be low. Therefore, it is a challenge for improving a residual stress to change a residual stress applied to the inner surface in the vicinity of the welded part to a compressive residual stress even when the heating temperature is low.
The object of the present invention is to provide a method for improving a residual stress and a method for construction management capable of changing a tensile residual stress applied to the inner surface in the vicinity of the welded part of a pipe to a compressive residual stress at a low construction temperature.